1. Field of the Invention
The present invention relates to a positioning signal receiving apparatus for receiving a plurality of positioning signals and determining the location of a point of signal reception.
2. Description of the Related Art
Conventionally, a positioning signal receiving apparatus, such as a GPS receiver for receiving signals transmitted by the Global Positioning System (GPS), receives radio waves transmitted from a plurality of positioning satellites, obtains GPS time and pseudo-ranges from a point of signal reception to the individual positioning satellites based on the phase of Coarse Acquisition (C/A) code, navigation messages, and so on superimposed on the radio waves, and determines the position of the point of signal reception from the locations of and the pseudo-ranges to the individual positioning satellites as well as time in a geodetic datum in which the positioning signal receiving apparatus is located.
Typically, this kind of GPS receiver continuously performs position fixing operation with the GPS receiver kept always powered on. In the case of a mobile positioning apparatus powered by a small-capacity battery or a positioning apparatus for conducting fixed-point observations using a combination of a solar cell and a secondary battery, for example, it is common practice to intermittently operate the apparatus by turning on the power only when it is necessary to obtain a position fix and turning off the power after obtaining the position fix to achieve as long an operating time as possible.
Under conditions where the positioning signal receiving apparatus is powered off, an internal counter of the receiving apparatus is not in operation. Therefore, at least a few seconds to a few tens of seconds are needed for the receiving apparatus to search for satellites, track carrier phases of satellite signals and acquire satellite information after power-on. Therefore, intermittent operation of the positioning signal receiving apparatus is associated with a problem that the apparatus can not quickly restart position fixing after power-on.
There exist conventionally known approaches to enabling a GPS receiver to estimate GPS time with high accuracy and quickly restart position fixing after power-on during intermittent operation. One such approach is to shut down a radio-frequency (RF) converter for receiving satellite signals and a digital signal processing circuit or mask (suspend) a clock signal of a central processing unit (CPU) to reduce power consumption while keeping a reference clock signal generator and a reference clock counter for estimating GPS time of the GPS receiver operational.
For example, Japanese Patent Application Publication No. 2002-6022 describes a method used in a GPS receiver, in which a reference clock signal generator used for estimating GPS time and a low-frequency clock signal generator used for measuring the lapse of “idle” (or “sleep”) time (during which the GPS receiver is not in full operation) and counting local time independently of the reference clock signal generator are kept operating even during the sleep time. The GPS receiver exactly estimates Week Number and Time of Week included in GPS time from clock pulses generated by the low-frequency clock signal generator used for local time determination.
In a case where the GPS receiver is operated intermittently as mentioned above, accuracy of estimating GPS time greatly depends on accuracy of the frequency of a clock signal generated during the sleep time. Conventional GPS receivers measure the lapse of sleep time by using a high-frequency oscillator which oscillates at a higher frequency than the aforementioned low-frequency clock signal generator, such as a temperature-compensated crystal oscillator which oscillates at a high frequency of 16.368 MHz, for instance, to satisfy the need for estimating GPS time with higher accuracy.
Generally, there is a positive correlation between oscillating frequency and power consumption of an oscillator. For example, the aforementioned temperature-compensated crystal oscillator which oscillates at such a high frequency as 16.368 MHz draws an electric current of approximately a few milliamperes, whereas a crystal oscillator with no temperature compensation which oscillates at a low frequency (e.g., 32 kHz) draws only a few hundred microamperes or less. It is therefore understood that a positioning signal receiving apparatus operated intermittently by using a high-frequency oscillator consumes a larger amount of electric power than a positioning signal receiving apparatus operated intermittently by using a low-frequency oscillator which oscillates at a relatively low frequency.
It is needed for the GPS receiver to estimate GPS time with high accuracy and quickly restart position fixing upon restoration of normal operation from sleep conditions as mentioned above. If the GPS receiver is operated intermittently by using a temperature-compensated crystal oscillator which oscillates at a high frequency to estimate GPS time with high accuracy immediately after power-on, however, there arises a problem that the GPS receiver draws a large amount of electric current even during the sleep time, thereby diminishing power consumption effect of the intermittent operation.